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      4 <title>Sources for time zone and daylight saving time data</title>
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      6 <style>
      7 pre {margin-left: 2em; white-space: pre-wrap;}
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     10 <body>
     11 <h1>Sources for time zone and daylight saving time data</h1>
     12 <p>
     13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     15 rules are controlled by individual
     16 governments. They are sometimes changed with little notice, and their
     17 histories and planned futures are often recorded only fitfully. Here
     18 is a summary of attempts to organize and record relevant data in this
     19 area.
     20 </p>
     21   <h3>Outline</h3>
     22   <nav>
     23     <ul>
     24       <li>The <code><abbr>tz</abbr></code> database product and process
     25 	<ul>
     26 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     27 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     28 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     29 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     30 	</ul>
     31       </li>
     32       <li>Uses of the <code><abbr>tz</abbr></code> database
     33 	<ul>
     34 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     35 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     36 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     37 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     38 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     39 	</ul>
     40       </li>
     41       <li>Related data
     42 	<ul>
     43 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     44 	  <li><a href="#maps">Maps</a></li>
     45 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     46 	</ul>
     47       </li>
     48       <li>Timekeeping concepts
     49 	<ul>
     50 	  <li><a href="#civil">Civil time concepts and history</a></li>
     51 	  <li><a href="#national">National histories of legal time</a></li>
     52 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     53 	  <li><a href="#precision">Precision timekeeping</a></li>
     54 	  <li><a href="#notation">Time notation</a></li>
     55 	  <li><a href="#see-also">See also</a></li>
     56 	</ul>
     57       </li>
     58     </ul>
     59   </nav>
     60 
     61 <section>
     62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     63 <p>
     64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     65 time zone database contains code and data
     66 that represent the history of local time
     67 for many representative locations around the globe.
     68 It is updated periodically to reflect changes made by political bodies
     69 to time zone boundaries and daylight saving rules.
     70 This database (known as <code><abbr>tz</abbr></code>,
     71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     72 is used by several implementations,
     73 including
     74 <a href="https://www.gnu.org/software/libc/">the
     75 <abbr title="GNU's Not Unix">GNU</abbr>
     76 C Library</a> (used in
     77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     78 <a href="https://www.android.com">Android</a>,
     79 <a href="https://www.freebsd.org">Free<abbr
     80 title="Berkeley Software Distribution">BSD</abbr></a>,
     81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     83 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     84 <a href="https://cygwin.com">Cygwin</a>,
     85 <a href="https://mariadb.org">MariaDB</a>,
     86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     87 <a href="https://www.mysql.com">MySQL</a>,
     88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     89 title="Web Operating System">webOS</abbr></a>,
     90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     92 <a href="https://www.apple.com/ios"><abbr
     93 title="iPhone OS">iOS</abbr></a>,
     94 <a href="https://www.apple.com/macos">macOS</a>,
     95 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     96 <a href="https://www.vmssoftware.com">Open<abbr
     97 title="Virtual Memory System">VMS</abbr></a>,
     98 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
     99 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    100 <p>
    101 Each main entry in the database represents a <dfn>timezone</dfn>
    102 for a set of civil-time clocks that have all agreed since 1970.
    103 Timezones are typically identified by continent or ocean and then by the
    104 name of the largest city within the region containing the clocks.
    105 For example, <code>America/New_York</code>
    106 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    107 <code>America/Phoenix</code> represents most of Arizona, which
    108 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    109 <code>America/Detroit</code> represents most of Michigan, which uses
    110 eastern time but with different <abbr>DST</abbr> rules in 1975;
    111 and other entries represent smaller regions like Starke County,
    112 Indiana, which switched from central to eastern time in 1991
    113 and switched back in 2006.
    114 To use the database on an extended <a
    115 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    116 title="Portable Operating System Interface">POSIX</abbr></a>
    117 implementation set the <code><abbr>TZ</abbr></code>
    118 environment variable to the location's full name,
    119 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    120 <p>
    121 Associated with each timezone is a history of offsets from
    122 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    123 Time</a> (<abbr>UT</abbr>), which is <a
    124 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    125 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    126 for timestamps after 1960 this is more precisely <a
    127 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    128 Universal Time</a> (<abbr>UTC</abbr>).
    129 The database also records when daylight saving time was in use,
    130 along with some time zone abbreviations such as <abbr>EST</abbr>
    131 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    132 </section>
    133 
    134 <section>
    135 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    136 <p>
    137 The following <a
    138 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    139 the latest release's two
    140 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    141 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    142 <pre><code>mkdir tzdb
    143 cd tzdb
    144 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    145 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    146 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
    147 gzip -dc tzdata-latest.tar.gz | tar -xf -
    148 </code></pre>
    149 <p>Alternatively, the following shell commands download the same
    150 release in a single-tarball format containing extra data
    151 useful for regression testing:</p>
    152 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
    153 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    154 </code></pre>
    155 <p>These commands use convenience links to the latest release
    156 of the <code><abbr>tz</abbr></code> database hosted by the
    157 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    158 of the <a href="https://www.iana.org">Internet Assigned Numbers
    159 Authority (IANA)</a>.
    160 Older releases are in files named
    161 <code>tzcode<var>V</var>.tar.gz</code>,
    162 <code>tzdata<var>V</var>.tar.gz</code>, and
    163 <code>tzdb-<var>V</var>.tar.lz</code>,
    164 where <code><var>V</var></code> is the version.
    165 Since 1996, each version has been a four-digit year followed by
    166 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    167 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    168 through <samp>zzz</samp>, and so on).
    169 Since version 1999g, each release has been distributed in
    170 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    171 ustar interchange format</a>, compressed as described above;
    172 older releases use a nearly-compatible format.
    173 Since version 2016h, each release has contained a text file named
    174 "<samp>version</samp>" whose first (and currently only) line is the version.
    175 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    176 and are also available in an
    177 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    178 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    179 less-secure protocol.</p>
    180 <p>Alternatively, a development repository of code and data can be
    181 retrieved from <a href="https://github.com">GitHub</a> via the shell
    182 command:</p>
    183 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    184 </code></pre>
    185 <p>
    186 Since version 2012e, each release has been tagged in development repositories.
    187 Untagged commits are less well tested and probably contain
    188 more errors.</p>
    189 <p>
    190 After obtaining the code and data files, see the
    191 <code>README</code> file for what to do next.
    192 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    193 machine-readable binary files, one for each location. The binary files
    194 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    195 specified by <a href="https://tools.ietf.org/html/8536">Internet
    196 <abbr>RFC</abbr> 8536</a>.
    197 The code also lets
    198 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    199 location.</p>
    200 </section>
    201 
    202 <section>
    203 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    204 <p>
    205 The <code><abbr>tz</abbr></code> code and data
    206 are by no means authoritative. If you find errors, please
    207 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    208 the time zone mailing list. You can also <a
    209 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    210 and browse the <a
    211 href="https://mm.icann.org/pipermail/tz/">archive of old
    212 messages</a>.
    213 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    214 discussions</a> and corresponding data changes can be
    215 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    216 </p>
    217 <p>
    218 If your government plans to change its time zone boundaries or
    219 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    220 advance, as this will coordinate updates to many cell phones,
    221 computers, and other devices around the world.
    222 The change should be officially announced at least a year before it affects
    223 how clocks operate; otherwise, there is a good chance that some
    224 clocks will operate incorrectly after the change, due
    225 to delays in propagating updates to software and data. The shorter
    226 the notice, the more likely clock problems will arise; see "<a
    227 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    228 the Timing of Time Zone Changes</a>" for examples.
    229 The <code><abbr>tz</abbr></code> data can represent planned changes
    230 far into the future, and a long-planned change can easily be reverted
    231 or otherwise altered with a year's notice before the change would have
    232 affected clocks.
    233 </p>
    234 <p>
    235 Changes to the <code><abbr>tz</abbr></code> code and data are often
    236 propagated to clients via operating system updates, so
    237 client <code><abbr>tz</abbr></code> data can often be corrected by
    238 applying these updates. With GNU/Linux and similar systems, if your
    239 maintenance provider has not yet adopted the
    240 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    241 the process by tailoring the generic instructions in
    242 the <code><abbr>tz</abbr> README</code> file and installing the latest
    243 data yourself. System-specific instructions for installing the
    244 latest <code><abbr>tz</abbr></code> data have also been published
    245 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
    246 <a
    247 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    248 <a
    249 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    250 title="International Components for Unicode">ICU</abbr></a>,
    251 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    252 JDK</a>,
    253 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    254 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    255 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    256 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    257 </p>
    258 <p>Since version 2013a,
    259 sources for the <code><abbr>tz</abbr></code> database have been
    260 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    261 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    262 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    263 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    264 title="linefeed">LF</abbr></a>,
    265 which can be modified by common text editors such
    266 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    267 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    268 <a href="https://www.vim.org">vim</a>.
    269 Specialized source-file editing can be done via the
    270 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    271 zoneinfo</a> package for <a
    272 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    273 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    274 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    275 Studio Code</a>.
    276 </p>
    277 <p>
    278 For further information about updates, please see
    279 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    280 Maintaining the Time Zone Database</a> (Internet <abbr
    281 title="Request For Comments">RFC</abbr> 6557). More detail can be
    282 found in <a href="theory.html">Theory and pragmatics of the
    283 <code><abbr>tz</abbr></code> code and data</a>.
    284 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    285 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    286 </p>
    287 </section>
    288 
    289 <section>
    290 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    291 <ul>
    292 <li>The article
    293 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    294 an encyclopedic summary.</li>
    295 <li><a href="tz-how-to.html">How to Read the
    296 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    297 database format.</li>
    298 <li><a
    299 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    300 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    301 database's style.</li>
    302 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    303 managing time in the internet</a> analyzes the database longitudinally.</li>
    304 </ul>
    305 </section>
    306 
    307 <section>
    308 <h2 id="web">Web sites using recent versions of the
    309 <code><abbr>tz</abbr></code> database</h2>
    310 <p>
    311 These are listed roughly in ascending order of complexity and fanciness.
    312 </p>
    313 <ul>
    314 <li><a href="https://time.is">Time.is</a> shows locations'
    315 time and zones.</li>
    316 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    317 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    318 <a href="https://www.worldclock.com">The World Clock</a>
    319 are time zone converters.</li>
    320 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    321 publishes <code><abbr>tzdb</abbr></code>-derived data in
    322 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    323 title="comma-separated values">CSV</abbr></a> and
    324 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    325 title="Structured Query Language">SQL</abbr></a> form.</li>
    326 <li><a
    327 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    328 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    329 <li><a
    330 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    331 Time in 1000 Places</a> uses descriptions of the values.</li>
    332 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    333 uses a pulldown menu.</li>
    334 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    335 timezone information for all countries</a>
    336 displays tables of <abbr>DST</abbr> rules.
    337 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    338 Worldwide</a> lets you sort zone names and convert times.</li>
    339 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    340 time map and a time converter.</li>
    341 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    342 calculates the current time difference between locations.</li>
    343 <li><a href="http://www.wx-now.com">Weather Now</a> and
    344 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    345 </ul>
    346 </section>
    347 
    348 <section>
    349 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    350 <ul>
    351 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    352 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    353 Distribution Service (tzdist) working group</a> defined <a
    354 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    355 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    356 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    357 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    358 transferring time zone data by reference.
    359 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    360 implementations</a> are available.
    361 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    362 mailing list</a> discusses possible extensions.</li>
    363 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    364 Internet Calendaring and Scheduling Core Object Specification
    365 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    366 covers time zone
    367 data; see its VTIMEZONE calendar component.
    368 The iCalendar format requires specialized parsers and generators; a
    369 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    370 (Internet <abbr>RFC</abbr> 6321) uses
    371 <a href="https://www.w3.org/XML/"><abbr
    372 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    373 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    374 (Internet <abbr>RFC</abbr> 7265)
    375 uses <a href="https://www.json.org"><abbr
    376 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    377 </ul>
    378 </section>
    379 
    380 <section>
    381 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    382 <p>Although some of these do not fully support
    383 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    384 distributions you can generally work around compatibility problems by
    385 running the command <code>make rearguard_tarballs</code> and compiling
    386 from the resulting tarballs instead.</p>
    387 <ul>
    388 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    389 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    390 program that compiles
    391 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    392 Vzic is freely
    393 available under the <a
    394 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    395 General Public License (<abbr
    396 title="General Public License">GPL</abbr>)</a>.</li>
    397 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    398 database conversion utility</a> is like Vzic, except for the <a
    399 href="https://dotnet.microsoft.com">.NET framework</a>
    400 and with a <abbr>BSD</abbr>-style license.</li>
    401 <li><a
    402 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    403 contains a script <code>parse_olson</code> that compiles
    404 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    405 modules. It is part of the Perl <a
    406 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    407 which is freely
    408 available under both the <abbr>GPL</abbr> and the Perl Artistic
    409 License. DateTime::TimeZone also contains a script
    410 <code>tests_from_zdump</code> that generates test cases for each clock
    411 transition in the <code><abbr>tz</abbr></code> database.</li>
    412 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    413 Database Parser</a> is a
    414 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    415 runtime library with <a
    416 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
    417 adopted by
    418 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    419 the current iteration of the C++ standard.
    420 It is freely available under the
    421 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    422 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    423 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    424 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    425 libraries for internationalization that
    426 has a compiler from <code><abbr>tz</abbr></code> source
    427 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    428 (mentioned <a href="#CLDR">below</a>)
    429 into an <abbr>ICU</abbr>-specific format.
    430 <abbr>ICU</abbr> is freely available under a
    431 <abbr>BSD</abbr>-style license.</li>
    432 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    433 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    434 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    435 title="Application Program Interface">API</abbr>s for use. It is
    436 freely available under the <abbr>MIT</abbr> license.</li>
    437 <li>Java-based compilers and libraries include:
    438 <ul>
    439 <li>The <a
    440 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    441 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    442 <a href="https://openjdk.java.net/">OpenJDK</a> and
    443 <a href="https://jdk.java.net/">Oracle JDK</a>.
    444 Although its source code is proprietary, its executable is available under the
    445 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    446 Timezone Updater License Agreement</a>.</li>
    447 <li>The <a
    448 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    449 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    450 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    451 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    452 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    453 and time <abbr>API</abbr></a> contains a class
    454 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    455 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    456 Java 8 <code>java.time</code>, which its users should migrate to once
    457 they can assume Java 8 or later. It is available under the <a
    458 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    459 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    460 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    461 are alternatives to TZUpdater. IANA Updater's license is unclear;
    462 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    463 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    464 time library for Android</a> and
    465 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    466 time and interval library for Java</a> compile
    467 <code><abbr>tz</abbr></code> source into a binary format.
    468 Time4A is available under the Apache License and Time4J is
    469 available under the <a
    470 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    471 General Public License (<abbr title="Lesser General Public
    472 License">LGPL</abbr>)</a>.</li>
    473 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    474 Java-based libraries.</li>
    475 </ul>
    476 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    477 time <abbr>API</abbr> for .NET</a>
    478 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    479 It is freely available under the Apache License.</li>
    480 <li>Many modern
    481 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    482 runtimes support <code><abbr>tz</abbr></code> natively via the
    483 <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
    484 This can be used as-is or with most of the following libraries,
    485 many of which also support runtimes lacking the <samp>timeZone</samp> option.
    486 <ul>
    487 <li>The <a
    488 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
    489 timezone polyfill</a>
    490 is freely available under a <abbr>BSD</abbr>-style license.</li>
    491 <li>The <a href="https://date-fns.org/">date-fns</a>
    492 library manipulates timezone-aware timestamps in browsers and
    493 in <a href="https://nodejs.org/en/">Node.js</a>.
    494 It is freely available under the <abbr>MIT</abbr> license.</li>
    495 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    496 minimalist replacement for the date and time API of
    497 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    498 manipulation library.
    499 It is freely available under the <abbr>MIT</abbr> license.</li>
    500 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    501 timezone support for the <samp>Intl</samp> API.
    502 It is freely available under the <abbr>MIT</abbr> license.</li>
    503 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    504 Moment.js plugin.
    505 It is freely available under the <abbr>MIT</abbr> license.</li>
    506 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    507 JavaScript library that supports date arithmetic that is time zone
    508 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    509 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    510 supports live <code><abbr>tzdb</abbr></code> updates,
    511 astronomical and atomic time, a command-line interface,
    512 and full <a
    513 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    514 Its companion <a
    515 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    516 can generate <abbr>TZif</abbr> and other files, and a companion website
    517 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    518 convert timestamps, view transition histories, and download code and data.
    519 It is freely available under the <abbr>MIT</abbr> license.</li>
    520 </ul>
    521 The proposed <a
    522 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
    523 objects</a> let programs access an abstract view of
    524 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    525 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    526 problematic <samp>Date</samp> objects</a> when working with dates and times.
    527 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    528 compiler from <code><abbr>tz</abbr></code> source into
    529 <a href="https://julialang.org/">Julia</a>. It is freely available
    530 under the <abbr>MIT</abbr> license.</li>
    531 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    532 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    533 title="Free Pascal Compiler">FPC</abbr></a>
    534 compiles from <code><abbr>tz</abbr></code> source into
    535 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    536 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    537 and <a
    538 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    539 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    540 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    541 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    542 <a href="https://www.python.org">Python</a>.
    543 It is freely available under a <abbr>BSD</abbr>-style license.
    544 In code that can assume Python 3.6 or later it is largely superseded; see <a
    545 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    546 The Fastest Footgun in the West</a>.</li>
    547 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    548 Ruby Timezone Library</a>
    549 compiles <code><abbr>tz</abbr></code> source into
    550 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    551 It is freely available under the <abbr>MIT</abbr> license.</li>
    552 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    553 Library</a> is
    554 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    555 library that compiles <code><abbr>tz</abbr></code> source into a time
    556 zone repository whose format
    557 is either proprietary or an <abbr>XML</abbr>-encoded
    558 representation.</li>
    559 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    560 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    561 source into text files, along with a runtime that can read those
    562 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    563 license.</li>
    564 </ul>
    565 </section>
    566 
    567 <section>
    568 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    569 <ul>
    570 <li>The <a
    571 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    572 Library</a>
    573 has an independent, thread-safe implementation of
    574 a <abbr>TZif</abbr> file reader.
    575 This library is freely available under the LGPL
    576 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    577 <li><a href="https://www.gnome.org">GNOME</a>'s
    578 <a href="https://developer.gnome.org/glib/">GLib</a> has
    579 a <abbr>TZif</abbr> file reader written in C that
    580 creates a <code>GTimeZone</code> object representing sets
    581 of <abbr>UT</abbr> offsets.
    582 It is freely available under the <abbr>LGPL</abbr>.</li>
    583 <li>The
    584 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    585 <code>baltzo::TimeZoneUtil</code> component contains a C++
    586 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    587 the Apache License.</li>
    588 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    589 library that translates between <abbr>UT</abbr> and civil time and
    590 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    591 License.</li>
    592 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    593 library that reads <abbr>TZif</abbr> files and converts
    594 timestamps from one time zone or format to another.
    595 It is used by <a href="https://secure.php.net"><abbr
    596 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    597 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    598 and <a href="https://www.mongodb.com">MongoDB</a>.
    599 It is freely available under the <abbr>MIT</abbr> license.</li>
    600 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    601 <abbr>TZif</abbr> file reader.</li>
    602 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    603 DateTime::TimeZone::Tzfile</a>
    604 is a <abbr>TZif</abbr> file reader written in Perl.
    605 It is freely available under the same terms as Perl
    606 (dual <abbr>GPL</abbr> and Artistic license).</li>
    607 <li>Python has a <a id="python-zoneinfo"
    608 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    609 class</a> that reads <abbr>TZif</abbr> data and creates objects
    610 that represent <code><abbr>tzdb</abbr></code> timezones.
    611 Python is freely available under the
    612 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    613 License</a>.
    614 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
    615 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    616 supplies TZif data if the underlying system data cannot be found;
    617 it is freely available under the Apache License.</li>
    618 <li>The
    619 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    620 library contains a Python tool that
    621 converts <abbr>TZif</abbr> data into
    622 <abbr>JSON</abbr>-format data suitable for use
    623 in its JavaScript library for time zone conversion. Dates before 1970
    624 are not supported.</li>
    625 <li>The <a
    626 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    627 package contains <a href="https://www.haskell.org">Haskell</a> code that
    628 parses and uses <abbr>TZif</abbr> data. It is freely
    629 available under a <abbr>BSD</abbr>-style license.</li>
    630 </ul>
    631 </section>
    632 
    633 <section>
    634 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    635 <ul>
    636 <li><a href="https://foxclocks.org">FoxClocks</a>
    637 is an extension for <a href="https://www.google.com/chrome/">Google
    638 Chrome</a> and for <a
    639 href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
    640 Toolkit</a> applications like <a
    641 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    642 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    643 It displays multiple clocks in the application window, and has a mapping
    644 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    645 It is freely available under the <abbr>GPL</abbr>.</li>
    646 <li><a href="https://golang.org">Go programming language</a>
    647 implementations contain a copy of a 32-bit subset of a recent
    648 <code><abbr>tz</abbr></code> database in a
    649 Go-specific format.</li>
    650 <li>Microsoft Windows 8.1
    651 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    652 data (mentioned <a href="#CLDR">below</a>) used by the
    653 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    654 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    655 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    656 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    657 <a id="System.TimeZoneInfo"
    658 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    659 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    660 the older, proprietary method of Microsoft Windows 2000 and later,
    661 which stores time zone data in the
    662 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    663 <a
    664 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    665 Tzid table</a> or <a
    666 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    667 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    668 to <code><abbr>tz</abbr></code> names.
    669 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    670 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    671 <li><a
    672 href="https://www.oracle.com/java/index.html">Oracle
    673 Java</a> contains a copy of a subset of a recent
    674 <code><abbr>tz</abbr></code> database in a
    675 Java-specific format.</li>
    676 </ul>
    677 </section>
    678 
    679 <section>
    680 <h2 id="other-dbs">Other time zone databases</h2>
    681 <ul>
    682 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    683 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    684 time zone history atlases
    685 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    686 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    687 Although these extensive atlases
    688 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    689 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    690 they are unreliable as Shanks appears to have
    691 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    692 sources and do not indicate which entries are guesswork.</li>
    693 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    694 its own <code>tztab</code>(4) format.</li>
    695 <li>Microsoft Windows has proprietary data mentioned
    696 <a href="#System.TimeZoneInfo">above</a>.</li>
    697 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    698 is another time zone database.</li>
    699 <li>The <a
    700 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    701 Schedules Information Manual</a> of the
    702 International Air Transport Association
    703 gives current time zone rules for airports served by commercial aviation.</li>
    704 </ul>
    705 </section>
    706 
    707 <section>
    708 <h2 id="maps">Maps</h2>
    709 <ul>
    710 <li>The <a
    711 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    712 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    713 <a href="https://www.cia.gov">US Central Intelligence
    714 Agency (<abbr
    715 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    716 zone map; the
    717 <a
    718 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    719 Library Map Collection</a>
    720 of the University of Texas at Austin has copies of
    721 recent editions.
    722 The pictorial quality is good,
    723 but the maps do not indicate daylight saving time,
    724 and parts of the data are a few years out of date.</li>
    725 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    726 with current time</a>
    727 has several fancy time zone maps; it covers Russia particularly well.
    728 The maps' pictorial quality is not quite as good as the
    729 <abbr>CIA</abbr>'s
    730 but the maps are more up to date.</li>
    731 <li><a
    732 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    733 much is time wrong around the world?</a> maps the difference between
    734 mean solar and standard time, highlighting areas such as western China
    735 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    736 </ul>
    737 </section>
    738 
    739 <section>
    740 <h2 id="boundaries">Time zone boundaries</h2>
    741 <p>Geographical boundaries between timezones are available
    742 from several <a
    743 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    744 geolocation</a>
    745 services and other sources.</p>
    746 <ul>
    747 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    748 Boundary Builder</a> extracts
    749 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    750 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    751 Its code is freely available under the <abbr>MIT</abbr> license, and
    752 its data entries are freely available under the
    753 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    754 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    755 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    756 <code><abbr>tzdb</abbr></code> timezones include:
    757 <ul>
    758 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    759 written in <a
    760 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    761 and is freely available under the <abbr>MIT</abbr> license.</li>
    762 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    763 is written in Go and is freely available under the Apache License.</li>
    764 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    765 in both Java and
    766 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    767 form, is freely available under the MIT license.</li>
    768 <li>For Node.js,
    769 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    770 is freely available under the MIT license, and
    771 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    772 is in the public domain.</li>
    773 <li>The <a
    774 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    775 library for Python is freely available under the MIT license.
    776 <li>The <a
    777 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    778 library for Ruby is freely available under the MIT license.</li>
    779 </ul></li>
    780 <li>Free access via a network API, if you register a key, is provided by
    781 the <a
    782 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    783 Timezone web service</a>, the <a
    784 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    785 Maps Time Zone API</a>, and
    786 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    787 Commercial network API access is provided
    788 by <a href="https://askgeo.com">AskGeo</a>
    789 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    790 </li>
    791 <li>"<a
    792 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    793 to get a time zone from a location using latitude and longitude
    794 coordinates?</a>" discusses other geolocation possibilities.</li>
    795 <li><a href="http://statoids.com/statoids.html">Administrative
    796 Divisions of Countries ("Statoids")</a> lists
    797 political subdivision data related to time zones.</li>
    798 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    799 zone boundaries for multizone countries</a> summarizes legal
    800 boundaries between time zones within countries.</li>
    801 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    802 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    803 world time zone boundaries distributed under the
    804 <abbr>GPL</abbr>.</li>
    805 <li>A ship within the <a
    806 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    807 waters</a> of any nation uses that nation's time. In international
    808 waters, time zone boundaries are meridians 15&deg; apart, except that
    809 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    810 wide and are separated by
    811 the 180&deg; meridian (not by the International Date Line, which is
    812 for land and territorial waters only). A captain can change ship's
    813 clocks any time after entering a new time zone; midnight changes are
    814 common.</li>
    815 </ul>
    816 </section>
    817 
    818 <section>
    819 <h2 id="civil">Civil time concepts and history</h2>
    820 <ul>
    821 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    822 Walk through Time</a>
    823 surveys the evolution of timekeeping.</li>
    824 <li>The history of daylight saving time is surveyed in <a
    825 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    826 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    827 <a href="http://seizethedaylight.com/dst/">A Brief
    828 History of Daylight Saving Time</a>.</li>
    829 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    830 Lords</a> discusses how authoritarians manipulate civil time.</li>
    831 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    832 contains guidelines and best practices for software applications that
    833 deal with civil time.</li>
    834 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    835 the International Date Line</a> tells the story of the most important
    836 time zone boundary.</li>
    837 <li><a href="http://statoids.com/tconcept.html">Basic Time
    838 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    839 </ul>
    840 </section>
    841 
    842 <section>
    843 <h2 id="national">National histories of legal time</h2>
    844 <dl>
    845 <dt>Australia</dt>
    846 <dd>The Parliamentary Library commissioned a <a
    847 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    848 paper on daylight saving time in Australia</a>.
    849 The Bureau of Meteorology publishes a list of <a
    850 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    851 Dates of Daylight Savings Time within Australia</a>.</dd>
    852 <dt>Belgium</dt>
    853 <dd>The Royal Observatory of Belgium maintains a table of time in
    854 Belgium (in
    855 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    856 hreflang="nl">Dutch</a> and <a
    857 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    858 hreflang="fr">French</a>).</dd>
    859 <dt>Brazil</dt>
    860 <dd>The Time Service Department of the National Observatory
    861 records <a href="http://pcdsh01.on.br/DecHV.html"
    862 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    863 Portuguese)</a>.</dd>
    864 <dt>Canada</dt>
    865 <dd>National Research Council Canada publishes current
    866 and some older information about <a
    867 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    868 zones and daylight saving time</a>.</dd>
    869 <dt>Chile</dt>
    870 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    871 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    872 Chile's official time (in Spanish)</a>.</dd>
    873 <dt>China</dt>
    874 <dd>The Hong Kong Observatory maintains a
    875 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    876  summer time in Hong Kong</a>,
    877 and Macau's Meteorological and Geophysical Bureau maintains a <a
    878 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    879 history for Macau</a>.
    880 Unfortunately the latter is incomplete and has errors.</dd>
    881 <dt>Czech Republic</dt>
    882 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    883 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    884 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    885 <dt>Germany</dt>
    886 <dd>The National Institute for Science and Technology maintains the <a
    887 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    888 of Legal Time in Germany</a>.</dd>
    889 <dt>Israel</dt>
    890 <dd>The Interior Ministry periodically issues <a
    891 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    892 hreflang="he">announcements (in Hebrew)</a>.</dd>
    893 <dt>Malaysia</dt>
    894 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    895 <dt>Mexico</dt>
    896 <dd>The Investigation and Analysis Service of the Mexican Library of
    897 Congress has published a <a
    898 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    899 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    900 <dt>Netherlands</dt>
    901 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    902 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    903 covers the history of local time in the Netherlands from ancient times.</dd>
    904 <dt>New Zealand</dt>
    905 <dd>The Department of Internal Affairs maintains a brief <a
    906 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    907 Daylight Saving</a>.</dd>
    908 <dt>Portugal</dt>
    909 <dd>The Lisbon Astronomical Observatory publishes a
    910 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    911 legal time (in Portuguese)</a>.</dd>
    912 <dt>Singapore</dt>
    913 <dd><a id="Singapore"
    914 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    915 is Singapore in the "Wrong" Time Zone?</a> details the
    916 history of legal time in Singapore and Malaysia.</dd>
    917 <dt>United Kingdom</dt>
    918 <dd><a
    919 href="https://www.polyomino.org.uk/british-time/">History of
    920 legal time in Britain</a> discusses in detail the country
    921 with perhaps the best-documented history of clock adjustments.</dd>
    922 <dt>United States</dt>
    923 <dd>The Department of Transportation's <a
    924 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    925 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    926 <dt>Uruguay</dt>
    927 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    928 Navy (SOHMA) publishes an annual <a
    929 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    930 (in Spanish)</a>.</dd>
    931 </dl>
    932 </section>
    933 
    934 <section>
    935 <h2 id="costs">Costs and benefits of time shifts</h2>
    936 <p>Various sources argue for and against daylight saving time and time
    937 zone shifts, and many scientific studies have been conducted. This
    938 section summarizes reviews and position statements based on
    939 scientific literature in the area.</p>
    940 <ul>
    941 <li>Carey RN, Sarma KM.
    942 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    943 daylight saving time on road traffic collision risk: a systematic
    944 review</a>.
    945 <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
    946 This reviews research literature and concludes that the evidence
    947 neither supports nor refutes road safety benefits from
    948 shifts in time zones.</li>
    949 <li>Havranek T, Herman D, Irsova D.
    950 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
    951 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
    952 2018;39(2).
    953 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    954 This analyzes research literature and concludes, "Electricity savings
    955 are larger for countries farther away from the equator, while
    956 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    957 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
    958 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
    959 an American Academy of Sleep Medicine position statement</a>.
    960 <em>J Clin Sleep Med.</em>
    961 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
    962 This argues for permanent standard time due to health risks of both
    963 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
    964 <li>Roenneberg T, Winnebeck EC, Klerman EB.
    965 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
    966 saving time and artificial time zones &ndash; a battle between
    967 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
    968 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
    969 This reviews evidence about the health effects of <abbr>DST</abbr>
    970 and concludes,
    971 "In summary, the scientific literature strongly argues against the
    972 switching between <abbr>DST</abbr> and Standard Time and even more so against
    973 adopting <abbr>DST</abbr> permanently."</li>
    974 </ul>
    975 </section>
    976 
    977 <section>
    978 <h2 id="precision">Precision timekeeping</h2>
    979 <ul>
    980 <li><a
    981 href="http://leapsecond.com/hpan/an1289.pdf">The
    982 Science of Timekeeping</a> is a thorough introduction
    983 to the theory and practice of precision timekeeping.</li>
    984 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    985 Time 2016</a> contains several freely-readable papers.</li>
    986 <li><a href="http://www.ntp.org"><abbr
    987 title="Network Time Protocol">NTP</abbr>: The Network
    988 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    989 discusses how to synchronize clocks of
    990 Internet hosts.</li>
    991 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    992 family of software algorithms can achieve accuracy to a few tens of
    993 nanoseconds in scalable server farms without special hardware.</li>
    994 <li>The <a
    995 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    996 Time Protocol</a> (<abbr
    997 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    998 can achieve submicrosecond clock accuracy on a local area network
    999 with special-purpose hardware.</li>
   1000 <li><a
   1001 href="https://tools.ietf.org/html/rfc4833">Timezone
   1002 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1003 (Internet <abbr>RFC</abbr> 4833)
   1004 specifies a <a
   1005 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1006 option for a server to configure
   1007 a client's time zone and daylight saving settings automatically.</li>
   1008 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1009 Scales</a> describes astronomical time scales like
   1010 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1011 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1012 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1013 <li>The <a href="https://www.iau.org"><abbr
   1014 title="International Astronomical Union">IAU</abbr></a>'s <a
   1015 href="http://www.iausofa.org"><abbr
   1016 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1017 collection contains C and <a
   1018 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1019 code for converting among time scales like
   1020 <abbr title="International Atomic Time">TAI</abbr>,
   1021 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1022 <abbr>UTC</abbr>.</li>
   1023 <li><a
   1024 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1025 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1026 diverse local time
   1027 scales used by each landed mission on Mars.</li>
   1028 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1029 dedicated not only to leap seconds but to precise time and frequency
   1030 in general. It covers the state of the art in amateur timekeeping, and
   1031 how the art has progressed over the past few decades.</li>
   1032 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1033 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1034 and time-signal emissions</a>, International Telecommunication Union &ndash;
   1035 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1036 <li><a
   1037 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1038 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1039 Bulletins</a> contains official publications of the International
   1040 Earth Rotation and Reference Systems Service, which decides when leap
   1041 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1042 via an optional "<code>right</code>" configuration where a computer's internal
   1043 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1044 as opposed to the default "<code>posix</code>" configuration
   1045 where the internal clock ignores leap seconds.
   1046 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1047 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1048 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1049 which corresponds to the first leap second
   1050 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1051 and to
   1052 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1053 In practice the two configurations also agree for timestamps before
   1054 1972 even though the historical situation is messy, partly because
   1055 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1056 is well-defined for sufficiently-old timestamps.</li>
   1057 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1058 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1059 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1060 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1061 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1062 "<code>posix</code>" configuration, is <a
   1063 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1064 the <abbr>NTP</abbr> reference implementation, <a
   1065 href="https://github.com/google/unsmear">supports</a> conversion between
   1066 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1067 cloud service providers. However, according to
   1068 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1069 Network Time Protocol Best Current Practices</a>
   1070 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1071 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1072 and is intended for use only in single, well-controlled environments.</li>
   1073 <li>The <a
   1074 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1075 Second Discussion List</a> covers <a
   1076 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1077 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1078 discussed further in
   1079 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1080 leap second: its history and possible future</a>.
   1081 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1082 might be redefined
   1083 without Leap Seconds</a> gives pointers on this
   1084 contentious issue, which was active until 2015 and could become active
   1085 again.</li>
   1086 </ul>
   1087 </section>
   1088 
   1089 <section>
   1090 <h2 id="notation">Time notation</h2>
   1091 <ul>
   1092 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
   1093 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1094 zone names, abbreviations, identifiers, and formats. For example, it
   1095 contains French translations for "Eastern European Summer Time",
   1096 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1097 "Bucharest". Its
   1098 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1099 charts</a> show these values for many locales. Data values are available in
   1100 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1101 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1102 <li>
   1103 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1104 the international standard date and time notation</a> covers
   1105 <a
   1106 href="https://www.iso.org/standard/70907.html"><em><abbr
   1107 title="International Organization for Standardization">ISO</abbr>
   1108 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1109 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1110 <li>
   1111 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1112 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1113 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1114 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
   1115 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1116 specifies the time notation used in email and <a
   1117 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1118 headers.</li>
   1119 <li>
   1120 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
   1121 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1122 specifies an <abbr>ISO</abbr> 8601
   1123 profile for use in new Internet
   1124 protocols.</li>
   1125 <li>
   1126 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1127 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1128 formats.</li>
   1129 <li>Alphabetic time zone abbreviations should not be used as unique
   1130 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1131 practice. For example, in English-speaking North America
   1132 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1133 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1134 and French-speaking North Americans prefer
   1135 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1136 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1137 database contains English abbreviations for many timestamps;
   1138 unfortunately some of these abbreviations were merely the database maintainers'
   1139 inventions, and these have been removed when possible.</li>
   1140 <li>Numeric time zone abbreviations typically count hours east of
   1141 <abbr>UT</abbr>, e.g., +09 for Japan and
   1142 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1143 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1144 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1145 title="Japan Standard Time">JST</abbr>-9"</code> and
   1146 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1147 for Japan and Hawaii, respectively. If the
   1148 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1149 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1150 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1151 confusion, handle old timestamps better, and insulate you better from
   1152 any future changes to the rules. One should never set
   1153 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1154 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1155 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1156 is called "<abbr>GMT</abbr>".</li>
   1157 </ul>
   1158 </section>
   1159 
   1160 <section>
   1161 <h2 id="see-also">See also</h2>
   1162 <ul>
   1163 <li><a href="theory.html">Theory and pragmatics of the
   1164 <code><abbr>tz</abbr></code> code and data</a></li>
   1165 <li><a href="tz-art.html">Time and the Arts</a></li>
   1166 </ul>
   1167 </section>
   1168 
   1169 <footer>
   1170 <hr>
   1171 This web page is in the public domain, so clarified as of
   1172 2009-05-17 by Arthur David Olson.
   1173 <br>
   1174 Please send corrections to this web page to the
   1175 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1176 </footer>
   1177 </body>
   1178 </html>
   1179